If the Prophet ﷺ was in fact the author of the Quran, there would be so many things that simply would not add up. Here are seven:
1 - Citing another woman by name multiple times (Sayyida Maryam) while not citing your own mother or wife. Maryam is cited by name 34 times! His own mother Amina: 0. Aisha: 0. Khadija: 0. If any man did this today, his family would have a serious reaction. But no, they knew he is not the author of this book.
2 - The story of the blind Ibn Umm Maktum. When the blind man disrupted an important meeting, the Prophet ﷺ was unhappy, but did not say anything so as not to offend this believer. He simply frowned silently. What happens next? It's revealed in the Quran that his face changed and was unhappy: the very thing the Prophet peace be upon him had tried to conceal! What author would do this?
3 - How about threatening yourself in the book your people recite publicly and daily: "If he (the Prophet) had attributed some fabrication to Us, We would certainly have seized his right hand and cut off his lifeblood, and none of you could have defended him" (69:44-47). Think about this: If your boss scolds you in a private email, would you then forward it to your staff? It would be a complete undermining of your authority. But no, the Sahaba knew this message is from Allah and the Prophet's authority is from Allah too, not from his own charisma and effort.
4 - Citing Prophet Musa nine times the amount of his own name (36 mentions in comparison to 4), while he is at war with the Jews and they are mocking him daily. They are in fact calling him an imitator who is copying them. If that happened to any of us, we would be sure to avoid citing the one we are accused of copying.
5 - Every author and every mover and shaker in the world is trying to be unique, original and bring something new to the world. Nobody wants to be a follower. But what does the Quran say? The exact opposite: "You are not anything new as a Messenger, say, 'I am nothing but a follower of what is revealed to me'" (46:9).
6 - How did the pagans mock the Prophet ﷺ? By saying he was abtar, or cut-off, not having any sons or lineage. He, peace be upon him, was very hurt by this. Normally, people bury what hurts them. They want to forget it. But in contrast, we have this verse: "Muhammad was not the father of any of your men" (33:40 Ahzab). If he peace be upon him was the author, why would he remind himself of this hurtful thing and then have everyone recite all the time?
7 What would a man say if you said, "You're trying to please your wife." Add to that: how would the Arabs of the 7th Century take it? Well this is exactly what happens in Surat al-Tahreem (66). An entire mini-drama and inter-family problem is put on open display in the initial verses of this sura. Two wives are jealous of the third, they devise a plot, it works...and the story goes on. If an imam made a post airing a family dispute in it, how would that go down?
Someone might reply, wow then His Lord was very mean to him, revealing these things like "cut off your blood line" and "you're not the father of men," and all the other verses. But that is not the case, for the Prophet ﷺ knew that these were all necessary for the sake of separating himself from any notion of authorship, just as many other hardships are necessary in the process of Prophethood.
And it proved true. The Quraysh called the Prophet many names, but eventually stopped calling him the author of the Quran. Abu Jahl himself admitted that it was a strategy that could not stand. Anyone who ponders this deeply will come to the same conclusion.
May Allah keep our hearts steadfast on trust, belief, and love of the Messenger peace be upon him.